We use fastapi in production too, but the problem we faced with sync stuff in combination with sqlalchemy was that the sessions (which we inject using Depends) were created before all the actual sync functions were executed, so the connection pool ran dry and everything became unresponsive. With flask I had a better experience because it creates the session in the same thread as the function that will handle the request. If you overload it a bit (say, 100 concurrent requests with a connection pool of 30) all the Depends calls will block because there are no threads left in the pool to actually handle the requests.
I understand that fastapi is more suited for async stuff for which it truly works great, but it would be nice if there was a idiomatic solution within fastapi and/or starlette that prevents these kind of problems.
Great work otherwise!